For gas turbine engines, it is generally known that the operational clearances between static engine structures and the tips of rotating blades impact the thermodynamic efficiency and fuel burn (e.g., specific fuel consumption or SFC) of the engine. Hence, gas turbine engine manufacturers continually seek ways to reduce these operational clearances, while at the same time avoiding rubs between the rotating blade tips and the static structure. The value of even several thousandths of an inch improvement can be quite significant.
Unfortunately, the lengths of the blade tips typically vary at a different rate than the static structures can expand or contract to accommodate the change in blade tip length, especially during transient operations. This can result in the blade tips contacting the static structure or cause excess clearance between the blade tips and static structure, both of which can reduce engine performance. One method that has been implemented to match the different growth rates is to supply a flow of air from the engine onto various rotor and/or static structures to reduce the operational clearances during steady state, high altitude cruise conditions. However, if a change in engine thrust is commanded while the operational clearances are reduced, rubs can unavoidably occur, resulting in damage and/or reduced engine performance.
Hence, there is a need for a system and method of controlling turbine blade clearances so that rubs between turbine blade tips and static structures is eliminated (or at least reduced) as a result of engine thrust level changes. The present invention addresses at least this need.